1. Field of the Invention
The present invention relates to standardized mechanical interface systems for reducing particle contamination of semiconductor wafers during semiconductor processing. More particularly, the present invention relates to a sealable, transportable container that has been adapted for horizontal loading and unloading.
2. Description of the Related Art
A standardized mechanical interface (SMIF) system has been proposed by the Hewlett-Packard Company as disclosed in U.S. Pat. No. 4,532,970 and 4,534,389. The purpose of the SMIF system is to reduce particle fluxes onto articles, for example, semiconductor wafers. This end is accomplished, in part, by mechanically ensuring that during transportation and storage the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
The SMIF concept is based on the realization that a small volume of controlled (with respect to motion, air flow direction and external contaminants), particle-free air provides a clean environment for wafers. Further details of one proposed system are described in the article entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulrich Kaempf, Solid State Technology, Jul. 1984, pp. 111-115. SMIF systems are concerned with particle sizes which range from below 0.02 .mu.m to above 200 .mu.m. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one half micron and under. Unwanted contamination particles which have geometries measuring greater than 0.1 .mu.m substantially interfere with 1 .mu.m geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.2 .mu.m and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafer cassettes; (2) canopies placed over cassette ports and wafer processing areas of processing equipment so that the environments inside the pods and canopies (after having clean air sources) become miniature clean spaces; and (3) a transfer mechanism to load/unload wafer cassettes from a sealed pod without contamination of the wafers in the wafer cassette from external environments.
One example of a conventional SMIF system is shown in FIGS. 1A and 1B, which illustrate a processing station 8 having a canopy 10 which is an easily removable shield that covers the wafer handling mechanism of processing equipment 12. Equipment 12 may be, for example, a photoresist applicator, mask aligner, inspection station or any similar processing equipment. The canopy 10, which is constructed of transparent plastic such as acrylic or polycarbonate to facilitate visual inspection and/or maintenance within the canopy 10, encloses the handling mechanism for the processing equipment 12 and a holder 14, such as a wafer cassette holding wafers 16. The environment within the processing equipment is separately maintained and separately cleaned: therefore, the equipment 12 need not be installed in a clean room.
A sealable transportable container (or pod) 18 including a box 20 having interior region 21 and a box door 32 is mounted on the horizontal surface 22 of a canopy 10 having a port assembly. The port assembly includes a port plate 24, port door 28, and an elevator assembly 30. Elevator assembly 30 transports a cassette 14, containing integrated circuit wafers 16 from the interior region 21 of a box 20 onto the region beneath the canopy 10.
In FIG. 1B, port door 28 and box door 32 are shown in the closed position by the dotted lines. A mover assembly 34 includes a platform 36, a shaft engagement device 38, and a drive motor 40. Platform 36, extending from elevator assembly 30, carries port door 28, box door 32 and cassette 14 in a vertical direction. Platform 36 is attached by engagement devices 38 to vertical guide 42 of elevator assembly 30.
Typically, guide 42 includes a lead screw (not shown) and the drive motor 40 drives a gear (not shown) which engages the lead screw for driving platform 36 up or down. When platform 36 is driven to the closed position, port door 28 closes the port opening in canopy 10. In a similar manner, a manipulator assembly shown generally by the numeral 44 is fastened to platform 46 which has an engagement means 48 for engaging vertical guide 42. Manipulator assembly 44 includes a manipulator arm 50 and engagement head 52 adapted to engage cassette 14. By vertical operation of the platforms 36 and 46 and by operation of the manipulator assembly 44, cassette 14 is moved from its position on box door 32 to a position on the equipment station 13 (as shown by the broken lines).
Although the SMIF system of FIGS. 1A and 1B has been successful in satisfying the need for a clean environment, areas of improvement have been identified for making the above-described system more convenient and efficient. For example, as can be seen in FIGS. 1A and 1B, a container is placed on top of the port. Typically, a port could be from 4 to 6 feet high. If the operator transporting the container is shorter than 6 feet, the operator may have trouble reaching the top of the port in order to position the container. Second, in order to remove a wafer from the container, the cassette must be lowered out of the container, the cassette must be moved to a work area and the wafer then removed from the cassette. Lowering and moving the cassette adds extra time to the semiconductor fabrication process and increases the risk of harm to the wafer due to the extra movement. It would be advantageous if the cassette can be easily and quickly removed from the container or if the wafer can be removed from the container without removing the cassette from the container. Third, in some applications a gas, such as nitrogen, is injected into the container. A cassette in the conventional container could partially block gas from entering at the bottom of the container. Finally, there is a need to have a retainer that holds the wafer and cassette stationary that is easy to manufacture and convenient to use.